1. Field of the Invention
The present invention generally relates to digital-to-analog converting circuits, and more particularly to a digital-to-analog converting circuit for digital audio circuits and the like.
2. Prior Art
In digital audio circuits such as a playback circuit of a compact disk, it is very important to reduce a distortion level in a playback processing of a minute signal as much as possible. In a conventional digital-to-analog converting circuit (hereinafter, referred to as a D/A converting circuit), pure binary data are converted into an analog signal by use of a weighted resistor group or a R-2R resistor ladder and the like. However, such conventional D/A converting circuit suffers a problem in that the distortion level in the processing of the minute signal must be relatively large. In order to reduce such distortion level, a floating point type D/A converting circuit is proposed. In this case, floating point data are applied to the D/A converting circuit wherein the floating point data are converted into an analog signal. Such D/A converting circuit is advantageous in that it is possible to reduce the distortion level in the processing of the minute signal. However, it is disadvantageous that the input data must be divided into an exponent-part and a mantissa-part. Hence, it is difficult to apply the pure binary data into the floating point type D/A converting circuit.
Therefore, the applicant of the present invention proposed a digital-to-analog converting circuit (i.e., Japanese Patent Application Preliminary Publication No. Sho61-87431) wherein the pure binary data can be applied thereto and the distortion level in the processing of the minute signal can be reduced. This proposed D/A converting circuit is one type of the floating point type D/A converting circuit wherein a bit shift operation is performed on the input data thereof in response to the value of the input data.
In the above D/A converting circuit, the bit shift operation is performed frequently when the value of the input data is varied frequently. Hence, a switching operation must be performed frequently in a switching circuit provided within the D/A converting circuit. Due to such frequently switching operation, a distortion depending on the switching operation must be generated frequently and such distortion deteriorates a quality of a reproduced tone if a difference between circuit. constants at times before and after a switching timing occurs or the input signal includes a DC offset level, for example.